Skyrmion birth at the notch

Abstract

Magnetic skyrmions are swirls of the spin texture, which can exist in certain magnetic materials. They may act as information carriers in future spintronic applications1,2 because of their robustness against perturbation — they are topologically protected — and because they can be manipulated with low currents. Yet, most such tech applications would require a simple means to create, manipulate and delete the information carriers individually. Back in 2013, Junichi Iwasaki et al.3 and João Sampaio et al.4 independently proposed two alternative routes to create and drive skyrmions in tracks of nanoscale dimensions based on electrical current injection. This sequence of images, taken from the work of Iwasaki and co-workers3, shows snapshots of the creation process based on the injection of an in-plane electrical current pulse into a ferromagnetic nanoscale strip with a notch. At time zero (a), all spins in the interior of the track point out of the plane (blue), while at the edges the spins possess an in-plane orientation (white) due to a subtle balance of the different magnetic interactions at play in the material. The electrical current pulse induces a spin-transfer torque, which pushes the spin configuration at the notch into the interior of the stripe (b–d) and creates a region with a spin component of opposite out-of-plane orientation (red). With time, this region grows and then detaches from the notch. A skyrmion is created (e,f). Now, low currents can move the skyrmion along the track. A repulsive force from the edges confines the skyrmion to the stripe of magnetic material and the polarity of the current determines the direction of motion. A large current pulse can then annihilate the skyrmion at will. It provides sufficient energy such that the skyrmion can overcome the repulsion at the boundary and merges into the edge. Sampaio et al.4 proposed an alternative mechanism for the creation of individual skyrmions, the local, vertical injection of a spin-polarized current into a thin-film ferromagnet. Spin-transfer torque locally inverts the magnetization and creates a skyrmion, which can then again be manipulated with in-plane currents. A low-temperature scanning tunnelling microscopy experiment realized this concept of skyrmion creation earlier that year5. These two earlier proposals triggered a plethora of experiments in the following years. Scientists first realized one or two of the components, the controlled creation, manipulation, or annihilation of individual skyrmions. Later experiments combined all in a single experiment or aimed at room temperature implementation or for alternative material systems such as ferrimagnets or artificial antiferromagnets. The appeal of the proposed schemes comes from their simplicity and their attractiveness for future tech applications. The twoor three-terminal control of currents is easy to handle and compatible with current technology. The low current densities needed for manipulation make these schemes energy efficient. ❐